A MEETING WITH THE UNIVERSE

Chapter 4-5

The End of Stars:Death and Transfiguration

White dwarfs

Perhaps the greatest surprise of the
Space Age has been the realization
that "dead" stars that have used all
their nuclear fuel can sometimes produce
more energy than they did when
"alive". We have discovered that there
are three possible ends for a burnt-out
star. If the star has about the mass of
the Sun, it will collapse under its own
gravity until the collective resistance
of the electrons within it finally halts
the process. The star has become a
white dwarf and may be comparable
in size to the Earth. A star with a mass
of about 1.5 to 2 or 3 times that of our
Sun will collapse even further, ending
up as a neutron star, perhaps 20 kilometers
in diameter. In neutron stars,
the force of gravity has overwhelmed
the resistance of electrons to compression
and has forced them to combine
with protons to form neutrons. Even
the nuclei of atoms are obliterated in
this process, and finally the collective
resistance of neutrons to compression
halts the collapse. At this point, the
star's matter is so dense that each
cubic centimeter weighs several
billion tons. For stars that end their
life weighing more than a few times
the mass of the Sun, even the resistance
of neutrons is not enough to stop
the inexorable gravitational collapse.
The star ultimately becomes a black
hole, a region in space so massive that
no light or matter can ever escape
from it.

The existence of white dwarfs
has been known for some time, and
many have been detected with
ground-based telescopes. However,
neutron stars and black holes existed
only in much-disputed theory until
the Space Age.

Strange remains of a shattered star.
Result of a supernova explosion seen in the year 1054 A.D.,
the Crab Nebula is now about 10 lightyears in diameter.
The Crab is shown in visible light;
filamentary structures are shreds of the disrupted star, while the
smooth while glow is radiation from high-speed electrons streaming
through a magnetic field in the nebula.

Strange remains of a shattered star. (cont.)
Two X-ray
images from HEAO-2 show the pulsar at the heart of the nebula as it
seems to blink on and off. Actually, the pulsar is a neutron star (the
surviving core of the exploded star), rotating 30 times per second,
each of its twin "searchlight" beams sweeping past the Earth at like
intervals. Each sweep corresponds to an observed pulse of X-rays,
gamma rays, visible light, and radio waves. The spinning core is
gradually slowing as it supplies energy to the fast electrons that make
the smooth part of the nebula shine.

Strange remains of a shattered star. (cont.)
Two
black-and-white photographs from the 5-meter (200-inch) Hale reflector on Mt.
Palomar are combined to reveal the motion of the filaments thrown
out in the 1054 A.D. explosion. A photo made in 1950 is printed as a
positive (bright regions are white), while one made in 1964 is printed
as a negative (bright regions are dark). Note that each small white
structure has a black rim on the outer side, indicating that
expansion from the center persists.

Neutron stars and supernovae

The discovery and understanding of
neutron stars involve studies of two
poorly understood types of space objects,
supernovae and pulsars. Supernovae
are extremely violent explosions,
in which a star suddenly detonates,
pouring out so much energy
that for a few days it may outshine all
the other stars in its galaxy put
together. Pulsars, first detected by
radio astronomers in 1967, are
sources of very accurately spaced
bursts of radio waves. These bursts
were so regular, in fact, that the scientists
who detected them wondered
briefly if they had found artificially
generated signals from an interstellar
civilization.

The discovery of a pulsar in the
Crab Nebula supernova remnant led
to a great synthesis in our understand
ing of pulsars and supernovae. Supernovae
occur at the end of a massive
star's life, when it is a red supergiant,
with its nuclear fuel almost spent.
When the central core becomes so
dense that electrons and protons
begin to form neutrons, it collapses
catastrophically to form a neutron
star. In the process, more energy is
released than the star ever generated
from its nuclear fuel, producing an
explosion in which every atom in the
outer parts of the star is heated to
well over a million degrees. The star is
literally destroyed in an instant, but
the debris from the explosion shines
briefly with the energy of a billion
suns.

Besides splattering stellar debris
into space, supernova explosions leave
behind a "cinder" - the dense, collapsed
core, made of neutrons - where
there once was a star. The weak magnetic
field of the original star is
greatly enhanced in the collapse, and
the remnant core - the neutron star -
may have a magnetic field trillions of
times stronger than the magnetic field
of the Earth. The rotation of the star
also increases dramatically during collapse,
and the resulting neutron star
spins many times a second. Beams of
radio waves, X-rays, and other radiation,
perhaps focused by the powerful
magnetic field, sweep through
space like the revolving beam of a
lighthouse. The neutron star has
become a pulsar.

Pulsars were discovered accidently
during a study of "twinkling" radio
sources in the sky. This twinkling is
not due to our atmosphere, as is the
twinkling of stars. Instead it is caused
by the highly rarefied interstellar gas,
which affects the passage of radio
waves. As the study went on, the scientists
at Cambridge University
noticed that in some sources the
twinkling was periodic, the signals
came at regular intervals of 1 or 2
seconds or less.

Gradually, more pulsars were discovered.
The fastest one known so
far, which rotates at 30 times a second
is in the Crab Nebula, the remnant of
a supernova explosion that was observed
in 1054 A.D. When this rapid
pulsar was found, it was quickly realized
that it must be a neutron star.
Only a neutron star could remain intact
under such rapid rotation with
out breaking up. (A rotating black hole
would remain intact, but it would not
produce a regular signal.)

Now that we can see the universe
by the light of X-rays and gamma rays,
further unexpected properties of pulsars
have been found. The theories
that were rather successful in explaining
the Crab Nebula pulsar failed to
predict or account for phenomena
found in the brightest gamma ray pulsar,
located in the constellation Vela.
New theories are needed to explain
how pulsars can create intense radio
waves, visible light, X-rays, and
gamma rays, all at the same time.
Many neutron stars of another
kind have been found with orbiting
X-ray telescopes. We usually cannot
detect the heat left over from their
collapse, but instead we detect X-rays
from matter that is heated intensely
as it falls rapidly towards the surface
of the star. The realization that neutron
stars suck up surrounding matter
came from the discovery in 1971 of an
X-ray pulsar, Hercules X-1. Detailed
study of this X-ray source revealed
very small variations in the 1.2
second period of pulsation. More study
proved that these small variations
were caused by motion of the neutron
star in orbit around another star.
We have now learned that most
X-ray emitting neutron stars are in
orbit around other, otherwise normal
stars. In some cases the stars are so
close that the intense gravity of the
neutron star actually pulls gas away
from the atmosphere of its companion.

Even when the stars are farther apart,
the neutron stars may collect material
from the stellar winds of the companions.
As the gas is pulled from the
normal star down to the surface of the
neutron star, the gravitational energy
of the neutron star heats the gas to
millions of degrees. The hot gas gives
off X-rays that mark for us the location
of the otherwise invisible neutron
star. X-ray pulsars derive their energy
from the accretion of matter; the pulsars
discovered by the radio astronomers
are mostly single stars that are
using up their energy of rotation and
thus are gradually slowing down.

Black holes: the end point

When the gravity of a collapsing star
is too strong for even neutrons to resist,
a black hole may be formed. A
black hole is a point mass in space,
surrounded by a literally black region
in which the gravity is so strong that
no matter, nor even light, can escape
it. But, just as in the case of a neutron
star, matter that falls toward the black
hole is intensely heated, producing
copious X-rays that can be detected
with telescopes flown above the
atmosphere.

A few of the brightest X-ray
sources in our galaxy are probably
black holes orbiting closely with relatively
ordinary stars. The X-ray
source called Cygnus X-1 is a famous
example. In 1971, astronomers
learned that Cygnus X-1 was associated
with a visible star that also is a
radio source. This discovery is an important
example of how ground-based
optical and radio telescopes work in
consort with orbiting X-ray telescopes
to solve the problems of Space Age
astronomy. The identity of the stellar
companion was confirmed when both
the radio source and the X-ray source
were observed to change dramatically
and simultaneously in intensity. Observations
of the spectrum of the visible
star and its changes in velocity as
it and its X-ray source companion
followed their orbits led to an estimate
of the mass of the X-ray source.
This unseen star that does produce
X-rays appears to have at least six
times the mass of our Sun, much more
than can possibly be supported by
the resistance of neutrons. Comparing
the deduced mass with the theoretical
limits on the masses of neutron
stars, we conclude that the unseen
X-ray source in the Cygnus X-1 binary
star system must be a black hole.
However, the proof necessarily is
limited - you can't see a black hole
and further studies of this and other
cosmic X-ray sources are needed.